Supramolecular Solid Complexes between Bis-pyridinium-4-oxime and Distinctive Cyanoiron Platforms.

The structural features and optical properties of supramolecular cyanoiron salts containing bis-pyridinium-4-oxime Toxogonin® (TOXO) as an electron acceptor are presented. The properties of the new TOXO-based cyanoiron materials were probed by employing two cyanoiron platforms: hexacyanoferrate(II), [Fe(CN)6]4- (HCF); and nitroprusside, [Fe(CN)5(NO)]2- (NP). Two water-insoluble inter-ionic donor-acceptor phases were characterized: the as-prepared microcrystalline reddish-brown (TOXO)2[Fe(CN)6]·8H2O (1a) with a medium-responsive, hydrochromic character; and the dark violet crystalline (TOXO)2[Fe(CN)6]·3.5H2O (1cr). Complex 1a, upon external stimulation, transforms to the violet anhydrous phase (TOXO)2[Fe(CN)6] (1b), which upon water uptake transforms back to 1a. Using the NP platform resulted in the water-insoluble crystalline salt TOXO[Fe(CN)5(NO)]·2H2O (2). The structures of 1cr and 2, solved by single-crystal X-ray diffraction, along with a comparative spectroscopic (UV-vis-NIR diffuse reflectance, IR, solid-state MAS-NMR, Mössbauer), thermal, powder X-ray diffraction, and microscopic analysis (SEM, TEM) of the isolated materials, provided insight for the supramolecular binding, electron-accepting, and H-bonding capabilities of TOXO in the self-assembly of these functionalized materials.

Evolutionarily conserved cysteines in plant cytosolic seryl-tRNA synthetase are important for its resistance to oxidation.

We have previously identified a unique disulfide bond in the crystal structure of Arabidopsis cytosolic seryl-tRNA synthetase involving cysteines evolutionarily conserved in all green plants. Here, we discovered that both cysteines are important for protein stability, but with opposite effects, and that their microenvironment may promote disulfide bond formation in oxidizing conditions. The crystal structure of the C244S mutant exhibited higher rigidity and an extensive network of noncovalent interactions correlating with its higher thermal stability. The activity of the wild-type showed resistance to oxidation with H2 O2 , while the activities of cysteine-to-serine mutants were impaired, indicating that the disulfide link may enable the protein to function under oxidative stress conditions which can be beneficial for an efficient plant stress response.

Zn(II) halide coordination compounds with imidazole and 2-methylimidazole. Structural and computational characterization of intermolecular interactions and disorder.

Novel zinc(II) coordination compounds with imidazole (Im) and 2-methylimidazole (2-MeIm) were prepared and characterized: [ZnX2(Im)2] (X = Cl (1a), Br (1b), I (1c)) and [ZnX2(2-MeIm)2] (X = Cl (2a), Br (2b), I (2c)). Coordination compounds 1a-c were prepared mechanochemically by neat grinding while 2a-c were prepared by solution synthesis. The complexes were characterized by FT-IR and NMR spectroscopy and by powder X-ray diffraction. Crystal and molecular structures were determined by the single crystal X-ray diffraction. The characteristic of all structures is a distorted tetrahedral coordination of zinc consisting of two halide atoms and two nitrogen atoms from the imidazole (or 2-methylimidazole) ligand. Molecules in 1a-c are interconnected by hydrogen bonds into 3D structures. Structures of 1b and 1c were found to have similar unit cells and similar crystal packing and hydrogen bonding. Introduction of the 2-methylimidazole substituent introduced disorder in the crystal structures of 2a-c. Because of the very small size of the crystals data were collected by synchrotron radiation. For the disordered 2a , 2b and 2c fixed geometry was used in refining of the structures. Crystal structures of 2a-c are characterized by chains of molecules connected by hydrogen bonds of the type N-H⋅⋅⋅X, with weak π⋅⋅⋅π and van der Waals interactions between the chains. The QTAIM, RDG and NCI computational analysis of 1a and 2a-c confirmed the presence of weak attractive intermolecular interactions that can be attributed to weak N-H⋅⋅⋅X and van der Waals interactions.

Copper Binding and Oligomerization Studies of the Metal Resistance Determinant CrdA from Helicobacter pylori.

Within this research, the CrdA protein from Helicobacter pylori (HpCrdA), a putative copper-binding protein important for the survival of bacterium, was biophysically characterized in a solution, and its binding affinity toward copper was experimentally determined. Incubation of HpCrdA with Cu(II) ions favors the formation of the monomeric species in the solution. The modeled HpCrdA structure shows a conserved methionine-rich region, a potential binding site for Cu(I), as in the structures of similar copper-binding proteins, CopC and PcoC, from Pseudomonas syringae and from Escherichia coli, respectively. Within the conserved amino acid motif, HpCrdA contains two additional methionines and two glutamic acid residues (MMXEMPGMXXMXEM) in comparison to CopC and PcoC but lacks the canonical Cu(II) binding site (two His) since the sequence has no His residues. The methionine-rich site is in a flexible loop and can adopt different geometries for the two copper oxidation states. It could bind copper in both oxidation states (I and II), but with different binding affinities, micromolar was found for Cu(II), and less than nanomolar is proposed for Cu(I). Considering that CrdA is a periplasmic protein involved in chaperoning copper export and delivery in the H. pylori cell and that the affinity of the interaction corresponds to a middle or strong metal-protein interaction depending on the copper oxidation state, we conclude that the interaction also occurs in vivo and is physiologically relevant for H. pylori.

Chemistry of Spontaneous Alkylation of Methimazole with 1,2-Dichloroethane.

Spontaneous S-alkylation of methimazole (1) with 1,2-dichloroethane (DCE) into 1,2-bis[(1-methyl-1H-imidazole-2-yl)thio]ethane (2), that we have described recently, opened the question about its formation pathway(s). Results of the synthetic, NMR spectroscopic, crystallographic and computational studies suggest that, under given conditions, 2 is obtained by direct attack of 1 on the chloroethyl derivative 2-[(chloroethyl)thio]-1-methyl-1H-imidazole (3), rather than through the isolated stable thiiranium ion isomer, i.e., 7-methyl-2H, 3H, 7H-imidazo[2,1-b]thiazol-4-ium chloride (4a, orthorhombic, space group Pnma), or in analogy with similar reactions, through postulated, but unproven intermediate thiiranium ion 5. Furthermore, in the reaction with 1, 4a prefers isomerization to the N-chloroethyl derivative, 1-chloroethyl-2,3-dihydro-3-methyl-1H-imidazole-2-thione (7), rather than alkylation to 2, while 7 further reacts with 1 to form 3-methyl-1-[(1-methyl-imidazole-2-yl)thioethyl]-1H-imidazole-2-thione (8, monoclinic, space group P 21/c). Additionally, during the isomerization of 3, the postulated intermediate thiiranium ion 5 was not detected by chromatographic and spectroscopic methods, nor by trapping with AgBF4. However, trapping resulted in the formation of the silver complex of compound 3, i.e., bis-{2-[(chloroethyl)thio]-1-methyl-1H-imidazole}-silver(I)tetrafluoroborate (6, monoclinic, space group P 21/c), which cyclized upon heating at 80 °C to 7-methyl-2H, 3H, 7H-imidazo[2,1-b]thiazol-4-ium tetrafluoroborate (4b, monoclinic, space group P 21/c). Finally, we observed thermal isomerization of both 2 and 2,3-dihydro-3-methyl-1-[(1-methyl-1H-imidazole-2-yl)thioethyl]-1H-imidazole-2-thione (8), into 1,2-bis(2,3-dihydro-3-methyl-1H-imidazole-2-thione-1-yl)ethane (9), which confirmed their structures.

The Solvent Effect on Composition and Dimensionality of Mercury(II) Complexes with Picolinic Acid.

Three new mercury(II) coordination compounds, {[HgCl(pic)]}n (1), [HgCl(pic)(picH)] (2), and [HgBr(pic)(picH)] (3) (picH = pyridine-2-carboxylic acid, picolinic acid) were prepared by reactions of the corresponding mercury(II) halides and picolinic acid in an aqueous (1) or alcohol-methanol or ethanol (2 and 3) solutions. Two different types of coordination compounds were obtained depending on the solvent used. The crystal structures were determined by the single-crystal X-ray structural analysis. Compound 1 is a one-dimensional (1-D) coordination polymer with mercury(II) ions bridged by chelating and bridging N,O,O'-picolinate ions. Each mercury(II) ion is four-coordinated with a bidentate picolinate ion, a carboxylate O atom from the symmetry-related picolinate ion and with a chloride ion; the resulting coordination environment can be described as a highly distorted tetrahedron. Compounds 2 and 3 are isostructural mononuclear coordination compounds, each mercury(II) ion being coordinated with the respective halide ion, N,O-bidentate picolinate ion, and N,O-bidentate picolinic acid in a highly distorted square-pyramidal coordination environment. Compounds 1-3 were characterized by IR spectroscopy, PXRD, and thermal methods (TGA/DSC) in the solid state and by 1H and 13C NMR spectroscopy in the DMSO solution.

Structural diversity in the coordination compounds of cobalt, nickel and copper with N-alkylglycinates: crystallographic and ESR study in the solid state.

Reactions of N-methylglycine (HMeGly), N-ethylglycine-hydrochloride (H2EtGlyCl) and N-propylglycine-hydrochloride (H2PrGlyCl) with cobalt(ii), nickel(ii) and copper(ii) ions in aqueous solutions resulted in ten new coordination compounds [Co(MeGly)2(H2O)2] (1), [{Co(MeGly)2}2(µ-OH)2]·2H2O (1d), [Cu(MeGly)2(H2O)2] (2α), [Co(EtGly)2(H2O)2] (3), [Ni(EtGly)2(H2O)2] (4), [Cu(µ-EtGly)2] n (5p), [Co(PrGly)2(H2O)2] (6), [Ni(PrGly)2(H2O)2] (7), and two polymorphs of [Cu(PrGly)2(H2O)2] (8α and 8ß). Compounds were characterized by single-crystal and powder X-ray diffraction, infrared spectroscopy, thermal analysis and X-band electron spin resonance (ESR) spectroscopy. These studies revealed a wide range of structural types including monomeric, dimeric and polymeric architectures, as well as different polymorphs. In all monomeric compounds, except 2α, and in the coordination polymer 5p hydrogen bonds interconnect the molecules into 2D layers with the alkyl chain pointing outward of the layer. In 2α and in the dimeric compound 1d hydrogen bonds link the molecules into 3D structures. 1d with cobalt(iii), and 4 and 7 with nickel(ii) are ESR silent. The ESR spectra of 1, 3 and 6 are characteristic for paramagnetic high-spin cobalt(ii). The ESR spectra of all copper(ii) coordination compounds show that the unpaired copper electron is located in the d x 2-y 2 orbital, being in agreement with the elongated octahedral geometry.

Binding of dipeptidyl peptidase III to the oxidative stress cell sensor Kelch-like ECH-associated protein 1 is a two-step process.

This work is about synergy of theory and experiment in revealing mechanism of binding of dipeptidyl peptidase III (DPP III) and Kelch-like ECH-associated protein 1 (KEAP1), the main cellular sensor of oxidative stress. The NRF2 ̶ KEAP1 signaling pathway is important for cell protection, but it is also impaired in many cancer cells where NRF2 target gene expression leads to resistance to chemotherapeutic drugs. DPP III competitively binds to KEAP1 in the conditions of oxidative stress and induces release of NRF2 and its translocation into nucleus. The binding is established mainly through the ETGE motif of DPP III and the Kelch domain of KEAP1. However, although part of a flexible loop, ETGE itself is firmly attached to the DPP III surface by strong hydrogen bonds. Using combined computational and experimental study, we found that DPP III ̶ Kelch binding is a two-step process comprising the endergonic loop detachment and exergonic DPP III ̶ Kelch interaction. Substitution of arginines, which keep the ETGE motif attached, decreases the work needed for its release and increases DPP III ̶ Kelch binding affinity. Interestingly, mutations of one of these arginine residues have been reported in cBioPortal for cancer genomics, implicating its possible involvement in cancer development. Communicated by Ramaswamy H. Sarma.

Cobalt, nickel and copper complexes with glycinamide: structural insights and magnetic properties.

Ten new compounds of Co, Ni and Cu with glycinamide (HL = glycinamide): [Co(H2O)2(HL)2]Cl2 (1a), [Co(H2O)2(HL)2]Br1.06Cl0.94 (1b), [Co(H2O)2(HL)2]I2 (1c), [Ni(H2O)2(HL)2]Cl2 (2a), [Ni(H2O)2(HL)2]Br0.94Cl1.06 (2b), [Ni(H2O)2(HL)2]I2 (low and room temperature polymorph, 2cLT and 2cRT), [CuCl2(HL)2] (3a), [CuBr1.3Cl0.7(HL)2] (3b) and {[Cu(HL)2]2[Cu2I6]} n (3c), as well as glycinamide hydroiodide (H2LI) and a new polymorph of glycinamide hydrochloride (ß-H2LCl) were prepared and characterized by single-crystal X-ray diffraction, infrared spectroscopy, thermal analysis (TG/DTA) and ESR spectroscopy. 1a, 1b, 2a and 2b are isostructural, as well as 1c and 2cRT, while the Cu compounds (3a-c) have entirely different molecular structures. All investigated compounds are mononuclear with exception of the 1D coordination polymer 3c. Compound 3c contains copper ions in the mixed oxidation state Cu(i) and Cu(ii) with interesting magnetic properties. Paramagnetic behaviour was found in 1a, 1b, 3a and 3b. Temperature induced polymorphic transformation was observed in 2c. Compounds 1a and 3a showed moderate antiproliferative activity and selectivity toward the human breast tumor cell line MCF-7.

Characterization of nZVI nanoparticles functionalized by EDTA and dipicolinic acid: a comparative study of metal ion removal from aqueous solutions.

The simultaneous adsorption of metal ions on bare and functionalized zero-valent iron nanoparticles (nZVI) from aqueous solution was tested using inductively coupled plasma optical emission spectrometry (ICP-OES). The nanomaterials were synthetized using borohydride reduction of iron salt followed by addition of EDTA and pyridine-2,6-dicarboxylic acid (dipicolinic acid, PDCA) in different molar ratios. Functionalized materials were characterized by FT-IR, XRD and SEM-EDS methods. The ligand attachment on the particles was confirmed by FT-IR spectroscopy. The formation of a magnetite and feroxyhyte shell on the core of functionalized nanoparticles was confirmed by the XRD study. Transformation of chain-like structures into clusters of nanospheres with smaller diameter size was observed from SEM study of EDTA-nVZI particles. The average diameter of bare nZVI particles comprised 115 nm, while EDTA functionalization resulted in an average diameter of 22 and 35 nm. The PDCA-nZVI particles obtained with the molar ratio of Fe : PDCA = 1 : 1 retain the chain-like structure with enlargement of the average particle diameter to 267 nm. SEM study of PDCA-nZVI particles that were produced using the ratio Fe : PDCA = 2 : 1 have demonstrated the unique property of elongation into ellipsoidal forms of reduced dimensions (a = 61 nm; b = 28 nm). The simultaneous metal ion removal from aqueous solution was the most efficient in the case of bare nZVI particles (91-97%). EDTA functionalization was found to be highly selective for Cu and Cr removal (95%), while PDCA functionalization shows selective adsorption of Cu, Cr and V in an aqueous medium (93%). Iron nanoparticles functionalized with PDCA in both of the used ratios showed more efficient metal ion adsorption in the case when smaller ellipsoidal particles were formed.

Arabidopsis seryl-tRNA synthetase: the first crystal structure and novel protein interactor of plant aminoacyl-tRNA synthetase.

The rules of the genetic code are established by aminoacyl-tRNA synthetases (aaRSs) enzymes, which covalently link tRNA with the cognate amino acid. Many aaRSs are involved in diverse cellular processes beyond translation, acting alone, or in complex with other proteins. However, studies of aaRS noncanonical assembly and functions in plants are scarce, as are structural studies of plant aaRSs. Here, we have solved the crystal structure of Arabidopsis thaliana cytosolic seryl-tRNA synthetase (SerRS), which is the first crystallographic structure of a plant aaRS. Arabidopsis SerRS displays structural features typical of canonical SerRSs, except for a unique intrasubunit disulfide bridge. In a yeast two-hybrid screen, we identified BEN1, a protein involved in the metabolism of plant brassinosteroid hormones, as a protein interactor of Arabidopsis SerRS. The SerRS:BEN1 complex is one of the first protein complexes of plant aaRSs discovered so far, and is a rare example of an aaRS interacting with an enzyme involved in primary or secondary metabolism. To pinpoint regions responsible for this interaction, we created truncated variants of SerRS and BEN1, and identified that the interaction interface involves the SerRS globular catalytic domain and the N-terminal extension of BEN1 protein. BEN1 does not have a strong impact on SerRS aminoacylation activity, indicating that the primary function of the complex is not the modification of SerRS canonical activity. Perhaps SerRS performs as yet unknown noncanonical functions mediated by BEN1. These findings indicate that - via SerRS and BEN1 - a link exists between the protein translation and steroid metabolic pathways of the plant cell. DATABASE: Structural data are available in the PDB under the accession number PDB ID 6GIR.

Structural and computational analysis of intermolecular interactions in a new 2-thiouracil polymorph.

The crystallization and characterization of a new polymorph of 2-thiouracil by single-crystal X-ray diffraction, Hirshfeld surface analysis and periodic density functional theory (DFT) calculations are described. The previously published polymorph (A) crystallizes in the triclinic space group P\overline{1}, while that described herein (B) crystallizes in the monoclinic space group P21/c. Periodic DFT calculations showed that the energies of polymorphs A and B, compared to the gas-phase geometry, were -108.8 and -29.4 kJ mol-1, respectively. The two polymorphs have different intermolecular contacts that were analyzed and are discussed in detail. Significant differences in the molecular structure were found only in the bond lengths and angles involving heteroatoms that are involved in hydrogen bonds. Decomposition of the Hirshfeld fingerprint plots revealed that O...H and S...H contacts cover over 50% of the noncovalent contacts in both of the polymorphs; however, they are quite different in strength. Hydrogen bonds of the N-H...O and N-H...S types were found in polymorph A, whereas in polymorph B, only those of the N-H...O type are present, resulting in a different packing in the unit cell. QTAIM (quantum theory of atoms in molecules) computational analysis showed that the interaction energies for these weak-to-medium strength hydrogen bonds with a noncovalent or mixed interaction character were estimated to fall within the ranges 5.4-10.2 and 4.9-9.2 kJ mol-1 for polymorphs A and B, respectively. Also, the NCI (noncovalent interaction) plots revealed weak stacking interactions. The interaction energies for these interactions were in the ranges 3.5-4.1 and 3.1-5.5 kJ mol-1 for polymorphs A and B, respectively, as shown by QTAIM analysis.

Structural characterization of FlgE2 protein from Helicobacter pylori hook.

The Helicobacter pylori flagellum is a complex rotatory nanomachine fundamental for the bacterium's survival in the human stomach. Protein FlgE is a component of the hook, a flexible junction exposed on the cell surface. In the H. pylori genome two different genes are present in different positions coding for hypothetical FlgE. The first protein, FlgE1, is the actual component of the flagellum hook, whilst the second, FlgE2, shares only 26% of the sequence identity with the other and its physiological function is still undefined. We have cloned, purified and crystallized FlgE2, whose structure, determined by the single-wavelength anomalous diffraction method, shows that in overall organization, the protein is composed of three distinct domains, two of them relatively similar to those of FlgE from other Gram-negative bacteria, whilst the third is peculiar to H. pylori. The crystal structure, along with the detected interaction with the regulatory cap protein FlgD, suggests a complementary function of FlgE1 and FlgE2 in the H. pylori flagellum, possibly typical of polar flagella, confirming the role of both proteins in the flagellar hook organization. Although some general features are shared with other Gram-negative bacteria, the presence of two different hook proteins indicates that the molecular organization of H. pylori flagellum has its own peculiarities. DATABASE: Atomic coordinates and structural factors have been deposited in the Protein Data Bank as 5NPY.

Stability data of FlgD from Helicobacter pylori and structural comparison with other homologs.

Flagellin component D (FlgD) from Helicobacter pylori is involved in the assembly of the hook of flagella, helical tubular structures that provide motility in non-filamentous bacteria. Data provided in this article refer to HpFlgD from strains 26695 (HpFlgD_26695) and G27 (HpFlgD_G27). Within this article, information on the secondary structure content and different type of interfaces found in the two crystal forms of HpFlgD (monoclinic, HpFlgD_m and tetragonal, HpFlgD_t) are provided, as well as the list of the hydrogen bonds between monomers that are relevant for their assembly into a tetramer. Additionally, data involving investigation of the size of HpFlgD in the solution and the crystallized HpFlgD are presented, "Crystal structure of truncated FlgD from the human pathogen Helicobacter pylori" [1]. The superposition of the different domains of HpFlgD (Fn-III and tudor domains) with the similar domains found in other species is shown, as well as the superposition of HpFlgD and modeled HpFlgE (flagellar hook protein).

Crystal structure of truncated FlgD from the human pathogen Helicobacter pylori.

Flagellin component D (FlgD) participates in the assembly of flagella, helical tubular structures that provide motility in non-filamentous bacteria. FlgD guides and controls the polymerization of FlgE that builds the hook, a short curved and hollow cylinder that connects the flagellar basal body spanning the cell envelope to the protruding filament. Crystal structures of truncated forms of Helicobacter pylori FlgD from two different strains in two space groups, I422 and P2, are reported here, at 2.2Å and 2.8Å resolution, respectively. Analogously to Pseudomonas aeruginosa and Xanthomonas campestris FlgD proteins, crystallization experiments set up for the full length protein resulted in crystals of a truncated form, lacking both N- and C-terminus ends. The crystal structures of the central domain show that the monomer is composed of a tudor and a fibronectin type III domain. The full length HpFlgD contains a long N-terminal signal region, probably partially flexible, a central globular region and a C-terminal segment with a peculiar repetitive pattern of amino acids. The spatial orientation of the two domains in HpFlgD differs from that of the homologous FlgD family members, P. aeruginosa and X. campestris. This difference together with the observation that HpFlgD assembles into tetramers, both in the solution and in the two crystal forms, strongly suggests that significant differences exist in the molecular organization of the flagella in different bacterial species.

Crystallographic snapshots of tyrosine phenol-lyase show that substrate strain plays a role in C-C bond cleavage.

The key step in the enzymatic reaction catalyzed by tyrosine phenol-lyase (TPL) is reversible cleavage of the Cß-Cγ bond of L-tyrosine. Here, we present X-ray structures for two enzymatic states that form just before and after the cleavage of the carbon-carbon bond. As for most other pyridoxal 5'-phosphate-dependent enzymes, the first state, a quinonoid intermediate, is central for the catalysis. We captured this relatively unstable intermediate in the crystalline state by introducing substitutions Y71F or F448H in Citrobacter freundii TPL and briefly soaking crystals of the mutant enzymes with a substrate 3-fluoro-L-tyrosine followed by flash-cooling. The X-ray structures, determined at ~2.0 Å resolution, reveal two quinonoid geometries: "relaxed" in the open and "tense" in the closed state of the active site. The "tense" state is characterized by changes in enzyme contacts made with the substrate's phenolic moiety, which result in significantly strained conformation at Cß and Cγ positions. We also captured, at 2.25 Å resolution, the X-ray structure for the state just after the substrate's Cß-Cγ bond cleavage by preparing the ternary complex between TPL, alanine quinonoid and pyridine N-oxide, which mimics the α-aminoacrylate intermediate with bound phenol. In this state, the enzyme-ligand contacts remain almost exactly the same as in the "tense" quinonoid, indicating that the strain induced by the closure of the active site facilitates elimination of phenol. Taken together, structural observations demonstrate that the enzyme serves not only to stabilize the transition state but also to destabilize the ground state.

Anion-templated supramolecular C3 assembly for efficient inclusion of charge-dispersed anions into hydrogen-bonded networks.

The binding properties and conformational adaptability of a known nitrate/sulfate receptor N,N'-3-azapentane-1,5-bis[3-(1-aminoethylidene)-6-methyl-3H-pyran-2,4-dione] (L) toward various charge-dispersed monoanions (HSO(3)(-), ClO(4)(-), IO(4)(-), PF(6)(-), and SbF(6)(-)) are considered. These anions template the folding of three HL(+) species through a self-assembly process into a new hollow supramolecular trication. During the self-assembly, all strong hydrogen-bond donors of the podand become coordinatively saturated by interactions with the oxo functionalities from other HL(+) molecules. In that way, only the weak hydrogen-bond-donating groups in the exterior part of the receptor are accessible for anion binding. The investigated anions are accommodated in the hydrophobic pockets of the isomorphous hydrogen-bonded frameworks, which serve as a basis for selective crystallization from the highly competitive anion/solvent systems. This behavior is discussed in terms of size and geometry of the anions as well as the receptor's coordination capabilities to provide the most favorable surroundings for guest inclusion both in solution and in the solid state.

Novel 9a,11-bridged azalides: One-pot synthesis of N'-substituted 2-imino-1,3-oxazolidines condensed to an azalide aglycone.

An efficient one-pot method for the synthesis of novel 9a,11-bridged 15-membered 9a-aza-9-deoxo-9a-homoerythromycin A and its 6-O-methyl analogue has been developed. The novel bicyclic azalide scaffold is characterized by an N'-substituted-2-imino-1,3-oxazolidine moiety bound to a macrolactone ring between positions 9a and 11. Removal of the cladinose sugar from the starting compounds allows easy preparation of a small series of such bicyclic 3-keto and 3,6-hemiketal azalide derivatives. A mechanism for the formation of N'-substituted-2-imino-1,3-oxazolidines is discussed. Antibacterial properties of the prepared compounds were evaluated.

Unusual azobenzene/bipyridine palladacycles: structural, dynamical, photophysical and theoretical studies.

Two types of Pd(ii) azobenzene/bipyridine complexes with unusual coordination mode of azobenzenes, PdCl{(mu-Cl)(mu-R(1)C(6)H(3)N=NC(6)H(3)R(2))}Pd(bpy) 1a-4a and [(bpy)PdCl(mu-NH(2)C(6)H(3)N=NC(6)H(4))Pd(bpy)]Cl 3b were formed by the reaction of dicyclopalladated azobenzenes (DMF)PdCl(mu-R(1)C(6)H(3)N=NC(6)H(3)R(2))PdCl(DMF) with excess of bpy, where bpy=2,2'-bipyridine; R(2)=H and R(1)=H (1), CH(3) (2), NH(2) (3) or R(1)=N(CH(3))(2) and R(2)=NO(2) (4). Neutral species 1a-4a were obtained in acetone, while in DMSO or MeOH the ionic complex 3b was produced. When dissolved, 3b decomposes to 3a and free bpy; however in DMSO upon addition of bpy 3b crystallizes again. X-ray structures of all complexes confirmed breaking of one Pd-N bond in the initial precursors, thus allowing rotation of one phenyl ring and positioning of both Pd atoms on the same side of the azobenzene ligand. Two Pd atoms are connected by the azobenzene ligand and in neutral complexes additionally by the Cl-bridge. In all complexes in the solid-state azobenzenes act simultaneously as monodentate C- and bidentate C,N-donors while bpy acts as bidentate donor. Variable-temperature (1)H NMR experiments established that structures of 1a-4a in DMF and DMSO at ambient temperature are not consistent with solid-state structures due to the fast exchange of one of the bpy nitrogen atoms bound to the Pd atom with solvent molecules. Theoretical studies confirmed the experimental structures as the most stable isomers. Photoabsorption and photoemission properties of the new complexes have been measured and photoabsorption is rationalized by time dependent DFT calculations. The presence of bpy significantly increases the intensity of fluorescence either in the solution (4a) or in the solid state (3a, 4a, 3b) at ambient temperature.